1. Field of the Invention
The present disclosure generally relates to microelectronic circuits and more particularly but not exclusively to an acoustic resonator intended to be integrated into a semiconductor product.
2. Field of the Invention
Acoustic resonators are well-known components that have been the subject of many studies. They are typically used for filtering operations, in particular in radio frequency (RF) filtering.
Conventionally, one distinguishes between Surface Acoustic Resonator (SAW) and Bulk Acoustic Resonators (BAW). In SAWs, the acoustic resonator is located on the surface of a semiconductor product while, in RAWs, it lays inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in this volume.
Acoustic resonators are frequently used in RF filtering. They are likely, however, to be useful in many other applications in the future but the principal obstacle to the use of acoustic resonators lies in the difficulty of integrating them into a semiconductor product.
Indeed, the characteristics of the components coming out at the end of production lines show great dispersions, and even BAW-type resonators that are yet the best ones suited for arrangement on a silicon substrate. Even with particularly ambitious and inevitably expensive specifications—for example, with a tolerance of about 1 percent for the dimensions of the resonator elements, dispersion on electric characteristics of the resonator cannot be eliminated.
Such a dispersion problem is typically settled by selectively selecting components so as to retain only those products from the production lines that comply with precise specifications.
This traditional approach cannot be chosen when aiming to integrate such an acoustic resonator on a substrate. Indeed, in this case, discarding a great number of manufactured products only because part of the product—which only accounts for a fraction of the added value of this product—does not show the characteristics specified in specifications, is quite out of the question.
This constitute a crippling obstacle to the direct integration of such acoustic components into an integrated circuit, this obstacle being likely to slow down the general use of acoustic resonators in RF applications and other ones too.
The following patents illustrate background art:
Work of reference “RF MEMS Circuit Design for Wireless Communications,” Hector J De Los Santos, Artech House, ISBN 1-58033 329-9, 2002, p. 163 and following, comprise general information on BAW-type acoustic resonators. This work does not address the problem of BAW resonator integration into an integrated circuit.
The integration of such components into an integrated circuit has been considered theoretically. Document “FBAR Filters at GHz Frequencies” by C Vale, J Rosenbaum, S. Horwitz, S. Krishnasvamy and R. Moore, in Forty Fourth Annual Symposium One Frequency Control, IEEE International Frequency Control Symposium, 1990, considers using on a same substrate, FBAR-type components in combination with passive elements in order to make filtering circuits. This document does not address the problem of the integration of these components into a semiconductor product; moreover, it does not describe how to increase the precision of the manufacturing method to allow such integration.
U.S. Pat. No. 5,446,306 entitled “Thin Film Voltage-Tuned Semiconductor Bulk Acoustic Resonator (SBAR)” describes tuning of a BAW resonator by means of a d.c. voltage, but does not actually describe how to proceed. Moreover, it completely ignores the problem of resonator integration into semiconductor products and, especially, how to adjust the frequency of the resonator in practical terms.
U.S. Pat. No. 5,714,917 entitled “Device Incorporating Has Tunable Thin Film Bulk Acoustic Resonator for Performing Amplitude and Phase Modulation” describes a BAW-type resonator which is made tunable in order to carry out phase and amplitude modulation. This document by no means describes how to adjust resonator frequencies nor does it show how to facilitate its integration into an integrated circuit while getting around the limitations inherent in the manufacturing method.
As can be noted, background art documents address neither the problem of the integration of acoustic resonators into a semiconductor product, nor how to remedy the manufacturing method's shortcomings.
This problem is however essential for whoever seriously aims at developing and using such components on a large-scale in future semiconductor products.